Conventionally, researches have been carried out on spinel ferrite which is a composite oxide containing iron as a magnetic material to be used as recording media of data-filing tapes. There are various compositions (M, Fe)3O4 (M=Fe(II), Co, Ni, Mn, Al, Cu, Zn, etc.) considered as the spinel ferrite, and one of the examples is Co ferrite. Co ferrite exhibits a large value of magnetocrystalline anisotropy constant so that it does not easily become superparamagnetic particles even if the particles become finer. Thus, it can maintain excellent magnetic properties and has been expected to be utilized as a high-density magnetic recording material.
Conventionally, a known method for preparing the aforementioned Co ferrite is to form it as a thin film by using a spin-spray-ferrite plating method. In order to further improve the magnetic properties of the magnetic material, a known research has been carried out in regards to forming Co-Ni ferrite thin films by the above-described method (see Zhang et al., “Ni Addition into Co Ferrite-plated Films”, Powder and Powder Metallurgy, vol. 47, No. 2, P 171-174, 25 of Feb., 2000).
Although Co—Ni ferrite which has been conventionally used is ferrimagnetic substance, it is a bulk material or of a thin film type. Thus, the coercivity is less than 239 [kA/m] (3000 [Oe]) even at the maximum, so that it is difficult to further improve the recording density of the recording medium using this magnetic material. That is, the spinel ferrites are used for tapes as recording media, however, the coercivity cannot be improved so that there raises an issue that the performance of the recording medium cannot be improved more than that of the present ones.
In the meantime, it has been examined to make finer particles of the magnetic material to be applicable for the recording media, and researches have been actively carried out on, for example, Ba ferrite as magnetoplumbite ferrite (M-type ferrite). However, although the magnetic particles with an average particle diameter of 30 [nm]-40 [nm] can be prepared, the coercivity still remains 239 [kA/m] (3000)[Oe]) at the maximum. Thus, the coercivity is still low.
That is, with the conventional magnetic materials, it is impossible to improve the coercivity more than that of the present level, so that it is difficult to achieve still higher recording density of the recording medium.
For overcoming the aforementioned shortcomings, presently, researches have been carried out in regards to Co—Ni spinel ferrite particles which, conventionally, had been the thin films or bulk materials. One of the examples is disclosed in a literature by H. Yamamoto, Y. Nissato, “Magnetic Properties of Co—Ni Spinel Ferrite Fine Particles with High Coercivity Prepared by the Chemical Coprecipitation Method”, IEEE Transaction on Magnetics, vol. 38, No. 5 pp 3488-3492, September 2002. In this literature, in order to improve the magnetic properties, especially, the coercivity of the conventional Co spinel ferrite fine particles, so as to provide a magnetic material with high coercivity HcJ, a part of Co is substituted with Ni for making Co coexists with Ni. Specifically, by setting the value of Fe/(Co+Ni) within a specific range, it is possible to obtain a magnetic material with relatively high coercivity.
However, with the above-described Co—Ni spinel ferrite, it is impossible to achieve small and uniform particle diameter, so that the coercivity of the particles also becomes nonuniform. Further, since there are a large percentage of superparamagnetic particles contained in the magnetic particles, the magnetic state varies according to the intensity of the surrounding magnetic field so that the magnetism cannot be stably maintained.
Therefore, when the magnetic fine particles of the above-described conventional case are used for a data storage recording medium, specifically for a magnetic tape, transcribing of recorded data is easily generated in a laminated area where the tape is being wound. That is, basically, if the coercivity is not uniform, the recorded data cannot be stably maintained in an area with low coercivity. Thus it is possible that the magnetic transcription is caused between magnetic tapes which overlap with each other. Moreover, in the area with the superparamagnetism, the magnetization intensity becomes unstable according to the magnetic state of the overlapping area of the magnetic tape so that the stable magnetic state cannot be maintained. As described above, when using the conventional magnetic fine particles for the magnetic recording media, the performance of recording information is deteriorated.
On the other hand, it is possible to remove the superparamagnetic substance from the generated magnetic fine particles in order to achieve the stable magnetic state. However, it requires time and work for this to be done, which increases the manufacturing cost.
Moreover, the values of saturation magnetization and remanent magnetization are not necessarily high. Thus, the magnetic properties are still insufficient to be used for the data storage recording media.
An object of the present invention is to improve the inconvenience of the conventional case described above and, specifically, to provide a magnetic material which can be preferably used for magnetic recording media, in which the content of the superparamagnetic fine particles is low while the high coercivity is maintained.